1. Field of the Invention
The present invention relates to a lens driving unit (driving unit for optical pick-up including lens) for driving a lens frame holding a lens in an optical axis direction by a screw feed mechanism of a lead screw and a nut screwed therewith.
2. Description of the Related Art
A well-known conventional lens driving unit includes a base (or a case), a lens frame holding a lens, a guide shaft fixed to the base so as to guide the lens frame in an optical axis direction, a driving motor fixed to the base, a lead screw directly connected to the driving motor and rotated and driven, a nut held unrotatably with respect to the lens frame (or its arm) and screwed with the lead screw, a coil spring urging the nut toward the lens frame or the lens frame toward the nut and the like. The lens frame is moved in the optical axis direction by rotating the lead screw by the driving motor so as to screw-feed the nut (see Unexamined Japanese Patent publication No. 10-20177, Unexamined Japanese Patent Publication No. 2004-341392, for example).
However, in the above conventional units, since a rotation stopper for the nut is directly engaged with a stopper of the lens frame, and an end face of the nut is held in direct contact (plane contact on the entire region) with a holding face of the lens frame (arm), vibration or the like is transmitted from the nut to the lens frame through the plane contact on the entire region between the nut end face and the holding face by assembly tolerance between the lead screw and the nut, load fluctuation caused by machining tolerance or the like of a screw portion formed on them, rotation driving force of the driving motor and its reaction and the like, which causes a problem of vibration of the lens frame in a direction perpendicular to the optical axis.
As a result, in an optical pick-up unit for CD or DVD to which this unit is applied, when the lens frame is to be moved in the optical axis direction during reading or writing of data on a disk, the optical axis of the lens is fluctuated and a focus on the disk can not be kept constant easily.